1. Field of the Invention
The present invention relates generally to electrical circuits, and more particularly but not exclusively to power factor correction.
2. Description of the Background Art
Power factor correction (PFC) is employed in power supply circuits to make the power supply appear as a purely resistive load as possible to the input AC (alternating current) line. In a nutshell, power factor correction allows current and voltage waveforms to be in-phase.
FIG. 1 shows an example switch mode power supply (SMPS) circuit 100 with continuous conduction mode (CCM) power factor correction (PFC). The power supply circuit 100 receives an input AC line voltage Vi and generates a power factor corrected output voltage Vo. The power supply circuit 100 is configured as a boost converter.
The power supply circuit 100 includes a voltage error amplifier circuit 103 that compares the output voltage VO against a reference voltage Vref to develop an error voltage indicative of the value of the output voltage VO. A multiplier 102 multiplies a sinusoidal reference 101 with the error voltage. For power factor correction, the sinusoidal reference 101 provides the requisite signal that is proportional to the rectified input AC line. A current error amplifier 104 compares the output of the multiplier 102 against the inductor current IL of the energy transfer inductor L to force the average current waveform to follow the voltage waveform. A PWM (pulse width modulation) modulator 105 compares the output of the current error amplifier 104 against a reference ramp signal to control switching of the drive transistor M to develop the output voltage VO in a way that the input line current Ii is in phase with the line voltage Vi, thereby maintaining a desired power factor of 1.0.
FIG. 2 shows the average of the inductor current IL, AVG in relation to the inductor current IL. FIG. 2 generally illustrates continuous conduction mode PFC. Note that unlike in discontinuous conduction mode PFC, the drive transistor M is controlled such that the inductor current IL does not reach zero during the switching cycle.
Also shown in FIG. 1 are a full-wave rectifier 106, an input capacitor Ci, an output diode D1, an output capacitor Co, and an output resistor Ro. As can be appreciated, the Power supply circuit 100 requires a multiplier, line sensing, and inductor current sensing for power factor correction.